Known aromatic polyesters or copolyesters, and especially polyethyleneterephthlate and its copolymers with small proportions of, for example, isophthalic acid or cyclohexanedimethanol, polybutyleneterephthlate and its copolymers, polytrimethyleneterephthalate, polyethylenenaphthalate and its copolymers serve as important starting materials for fibers, films, foils and packaging materials. These substances are processed following a melt condensation to granulates with a certain average viscosity. When reference is made to a polyester for the purposes of this application, that reference is intended to include all of these polyesters and copolyesters and derivatives thereof with similar crystallinity and physical properties.
The mean degree of polycondensation given in terms of intrinsic viscosity (I.V.) for polyethyleneterephthalates and its corresponding low modification copolyesters following melt condensation is in the range between 0.30 and 0.90 dl/g. The product is a partially crystalline granulate which can have a degree of crystallinity up to 9%.
In practice it is found that the production of a granulate with an intrinsic viscosity above 0.65 dl/g is scarcely possible in a conventional autoclave and that higher viscosities greater than about 0.80 dl/g result in a significant capacity reduction in melt polycondensation and, after stretching of, for example, a polyester for food packaging purposes, has a very low acetaldehyde content. As a consequence, a solid phase polycondensation (solid state polycondensation or SSP) when carried out to a predetermined degree of crystallization and can lead to an increase in the intrinsic viscosity in general by about 0.05 to 0.4 dl/g and a reduction of the acetaldehyde content from 25 to 100 ppm to values below 1 ppm in polyethyleneterephthalate (PET). The SSP reactor which is used for precrystallization and the final crystallization step can largely avoid caking in the reactor. The degree of crystallization, however, cannot be significantly raised while at the same time retaining the intrinsic viscosity and reducing the acetaldehyde content sufficiently.
Moreover the crystallization process precedes from the outside inwardly so that the degree of crystallinity is greater at the outer parts of the granule than at the center.
In an SSP reactor, the mean viscosity can be raised to enable the requisite strength for a corresponding application to be obtained while the acetaldehyde content for food packaging can be reduced based upon the requirement and the oligomer proportion is reduced to a minimum. It is also important that the acetaldehyde which is bound as a vinyl ester, can be broken down to the extent that the polyester granules can be readily worked into packaging material, for example, polyester bottles by a stretch blowing or injection blow molding process whereby there is only a minimum acetaldehyde content remaining in the polyester.
Processors for such granulates are usually manufacturers of such hollow bodies and containers.
In a preform machine utilizing an injection or extrusion process, parisons or so-called preforms can be produced which can be subsequently inserted into a blow mold and blown to the desired blow-molded shape. Other shaping processes can be used with such polyester granulates. For example, machines for film and foil production may also be used.
For granulation of synthetic resins, a strand granulation process has been developed and is on the market. This process produces a continuous relatively long plastic strand which is pressed through a perforated plate and the freely suspended plastic strands merging from the perforation can be cut off, for example, by air and passed through a water bath. Because of the relatively small surface area of a plastic strand by comparison to the granulate, the water pick up can be held to narrow limits. The cooled strands are dried and fed to a granulator. In this process, the granulation is effected in the solid state. In conjunction therewith a drying is effected, for example, as described in DE 43 14 162 or in conventional plastics handbooks. This granulation process can result in sharp point-like temperature increases in the strand and thus in enhanced decomposition effects in the polymer and a nonuniform decrease of crystallization from chip to chip. The cooling of the chip or particle is from the exterior inwardly.
A further granulation approach granulates the polymer melt following polycondensation by an underwater granulation whereby the melt, directly downstream of the nozzles or perforated plate of the granulate is subdivided in an adjacent water chamber with cutting blades. The subdivided granulate may still be plastic and deformable so as to be shaped by surface tension. Before it is quenched in cool water whereby again the cooling is effected from the exterior inwardly and the granulates assume a round or lens-shape contour, the cooled granulate is separated from the water stream in a water separator, is dried and then stored in Big Bags or in silos for further processing (see DE 35 41 500, DE 199 14 116, EP 0 432 427, DE 37 02 841). This process is termed the principle of drying with intrinsic heat. The chips or particles fabricated in this way has a uniform degree of crystallization which, however, is less than 10%.
In U.S. Pat. No. 4,436,782, a process for granulating and treating PET to produce pellets has been described in which an oligomer mixture with a viscosity of 0.08 to 0.15 is pressed through nozzles at a temperature between 260° C. and 280° C. so that droplets are produced which can pass through a cooling region with an inert gas atmosphere in a water bath or onto a transport belt so that the droplets will solidify into amorphous particles. In this method as well the pellets which are produced have a high proportion of amorphous structure.
In all of the described methods, the granulates which are obtained usually have a low degree of crystallinity, customarily less than 12%. In order to increase the crystallinity of the polymer granulate, for example as a step prior to SSP, cost-intensive reaction stages are known. High operating costs can thus result among other things because the granulate which is available at ambient temperature must initially be heated to the crystallization temperature.
Still another method and apparatus for treating thermoplastic polyesters and copolyesters to overcome at least some of the drawbacks of the above-described granulation process while obtaining a high degree of crystallinity are described in WO 01/81450. An abbreviated method of conventional granulation is described and utilizes known process steps and devices in order to produce surface crystallized droplet shaped intermediate products in the form of monomers, oligomers, monomer-glycol mixtures or partly polycondensed materials. The intermediate products are introduced into a gaseous medium, whereby the gaseous medium, after the introduction of the droplet intermediate, is accelerated in the gaseous medium of the crystallization stage and is thus passed through the crystallization stage in an accelerated manner. The droplet intermediate is maintained at a temperature greater than 100° C. but below its melting point for a certain period of time until sufficient crystallization is concluded at the surface of the particle. Here as well one obtains the greater crystallization in the outer layer. The result is a surface which is not sticky and promises to be capable of further treatment to a high polymer polycondensate.
The material made in this manner may not have the requisite mechanical strength. The particles tend to be more brittle by comparison with amorphous chips. A further disadvantage of this method of producing crystallinity in the lower molecular weight range is that in a closed SSP which can result in a crystallization of the chip through its entire cross section, the chip crystallinity can be destroyed in the melting process which produces the parison or preform by the injection molding step because of the nonuniform and high energy which is applied at that stage. In practice it is found that the melting temperature of the granulate may have to be a minimum of 300° C. which brings about a sharp increase in acetaldehyde formation in the preform and thus lowers the quality of the products which are produced because of increased decomposition reactions.
In practice, moreover, there is a danger that the SSP process can be hindered or even stopped by the immobility of the chain ends with increase in the viscosity.
Still another granulation process has been described in WO 01/05566 for the production of crystallized chips. In this case, synthetic resin strands of the molten plastic emerging from nozzles are partially crystallized as they pass directly into a controlled-temperature liquid medium along a crystallization stretch, the temperature in this liquid medium being maintained above the glass transition temperature of the synthetic resin strand. Following this crystallization, a granulating device subdivides the strands. The crystallization results in sufficient solidification and hardening at the periphery of the strand and enables the subdivision of the strand to granules only after a short temperature-controlled path within the granulating device and without prior drying of the strand. The result is pellets with a highly crystallized outer layer. A drawback of this process is that after granulation, a mixture of the granulate and the liquid medium is obtained which requires a drying of the granulate. That can be carried out conventionally.
A German patent document DE 103 49 016 describes a process for producing a plastic granulate which carries out an underwater granulation which rapidly separates the pellets from the water and allows them to dry and crystallize utilizing intrinsic heat. To avoid adhesion of the particles to one another, the pellets immediately following the centrifugal separation of them from the water, are caused to pass along a vibrating or oscillating conveyer over a sufficient residence time to the filling, further processing or packaging units, the crystallization process here does take place from the interior toward the exterior and enables a uniform crystallization to be obtained over the diameter of the granulate.
Such a process is referred to below as a latent heat crystallization process. Apart from the process described in DE 103 49 016, under the designation “latent heat crystallization process” falls also all conventional processes in which the crystallization is effected exclusively by use of the intrinsic heat from the molten state of the polymer. This means that the pellets between granulation and the subsequent packaging apparatus or further processing apparatus are not supplied with heat from the exterior. The avoidance of heat input from the exterior means that all of the media contacting the pellets must be either at the same temperature or a lower temperature than the temperature at the actual pellet surfaces. If the temperature of such media however is too low, excessive heat is withdrawn from the pellets and the desired latent heat crystallization can no longer occur to a sufficient degree. The basic principle of this process can be seen from DE 103 49 016.
A granulate produced by a latent heat crystallization process can have very different characteristics. These depend, apart from the operating conditions for the latent heat crystallization process, also upon the characteristics of the polymer melt and in the case of polyesters, for example, upon the degree of polymerization, the intrinsic viscosity (I.V.) and the acetaldehyde content. The characteristics of the product will be selected based upon the intended purpose of the latent heat crystallized granulate since they directly affect the subsequent processing step.